Process for preparing a high purity fischer-tropsch gasoil fraction

ABSTRACT

The present invention provides a process for preparing a high purity Fischer-Tropsch gasoil fraction, comprising: a)fractionating a Fischer-Tropsch-derived gasoil feedstock into two or more Fischer-Tropsch gasoil fractions having a different boiling point range, wherein at least one Fischer-Tropsch gasoil fraction is a contaminant-enriched Fischer-Tropsch gasoil fraction, which is enriched in one or more contaminants with respect to the feedstock; b) providing the contaminant-enriched Fischer-Tropsch gasoil fraction to an absorption zone comprising at least one absorbent material and contacting the contaminant-enriched Fischer-Tropsch gasoil fraction with the absorbent material to absorb at least part of contaminant; and c) retrieving from the absorption zone a purified Fischer-Tropsch gasoil fraction as high purity Fischer-Tropsch gasoil fraction, which purified Fischer-Tropsch gasoil fraction is contaminant-depleted with respect to the Fischer-Tropsch-derived gasoil feedstock, and wherein at least part of the contaminants in the contaminant-enriched Fischer Tropsch fraction are formed during the fractionation of step (a).

FIELD OF THE INVENTION

The present invention relates to a process for preparing a high purity Fischer-Tropsch gasoil fraction and use thereof as a solvent or functional fluid.

BACKGROUND TO THE INVENTION

In the last two decades there has been an increasing interest in synthetic paraffinic hydrocarbon products. Such synthetic paraffinic products are for instance produced by so-called Fischer-Tropsch processes, wherein synthesis gas, i.e. a mixture of predominantly hydrogen and carbon monoxide, is converted into higher hydrocarbon compounds including paraffins.

Synthetic paraffinic products of particular interest are Fischer-Tropsch-derived gasoils. Due to their synthetic origin, these Fischer-Tropsch-derived gasoils have very low levels of aromatics, naphthenics and impurities compared to their crude oil derived counterparts. In addition, the Fischer-Tropsch-derived gasoils have properties that provide advantages in solvent and functional fluid applications with low viscosity requirements.

US 2012/0048775 describes a process for producing middle distillates from a paraffinic feed produced by Fischer-Tropsch synthesis, wherein an intermediate fraction with an initial boiling point in the range 150° C. to 400° C. and an end point in the range 300° C. to 450° C. is passed over an ion exchange resin, and optionally decontaminated by passage over a guard bed. In U.S. Pat. No. 5,906,727, a Fischer-Tropsch-derived solvent is disclosed with a boiling range from approximately 160 to 370° C. According to U.S. Pat. No. 5,906,727, the solvent has a low odor and is colorless (Saybolt color number of +30).

There is a need in the art for Fischer-Tropsch-derived solvents that have more narrow boiling point range compared to the solvent disclosed in U.S. Pat. No. 5,906,727.

SUMMARY OF THE INVENTION

The present invention provides for a process for preparing a high purity Fischer-Tropsch gasoil fraction. It has been found that, when a Fischer-Tropsch-derived gasoil having a relatively broad boiling point range, e.g. within a range of from approximately 150 to 450° C., is fractionated into two or more fractions that have a more narrow boiling point range, at least one of the fractions exhibits an increased odor and/or discoloration. These disadvantageous side-effects encountered when fractionating virtually odorless and colorless Fischer-Tropsch-derived gasoil were till now unknown. It has now been found that this problem can be resolved by the process according to the present invention.

Accordingly, the present invention provides a process for preparing a purified Fischer-Tropsch gasoil fraction, comprising:

a) fractionating a Fischer-Tropsch-derived gasoil feedstock,

wherein said Fischer-Tropsch-derived gasoil is a fluid comprising paraffins, including isoparaffins and normal paraffins, with alkyl chain lengths in the range of from 7 to 30 carbon atoms, comprising at least 70 wt % of Fischer-Tropsch-derived paraffins having 9 to 25 carbon atoms based on the total amount of Fischer-Tropsch-derived paraffins,

into two or more Fischer-Tropsch gasoil fractions having a different boiling point range, wherein at least one Fischer-Tropsch gasoil fraction is a contaminant-enriched Fischer-Tropsch gasoil fraction, which is enriched in one or more contaminants with respect to the feedstock; b) providing the contaminant-enriched Fischer-Tropsch gasoil fraction to an absorption zone comprising at least one absorbent material and contacting the contaminant-enriched Fischer-Tropsch gasoil fraction with the absorbent material to absorb at least part of contaminant; and c) retrieving from the absorption zone a purified Fischer-Tropsch gasoil fraction as high purity Fischer-Tropsch gasoil fraction, which purified Fischer-Tropsch gasoil fraction is contaminant-depleted with respect to the contaminant-enriched Fischer-Tropsch gasoil fraction, and wherein at least part of the contaminants in the contaminant-enriched Fischer Tropsch fraction are formed during the fractionation of step (a).

The process according to the invention allows for the preparation of high purity Fischer-Tropsch gasoil fractions, having applicability as solvents, diluents and functional fluids, which have a boiling point range that is narrower than the Fischer-Tropsch-derived gasoil (also referred to as Fischer-Tropsch-derived gasoil) feedstock it was prepared from.

The process according to the invention further allows for the preparation of purified Fischer-Tropsch gasoil fractions, having applicability as solvents, diluents and functional fluids, which have desired odor properties and/or color specifications.

The process according to the invention further allows for the removal of contaminants using a relatively straightforward, cheap and safe absorption method, compared to for instance treatments with strong acids like concentrated sulfuric acid or elaborate and expensive hydrotreating treatments.

In another aspect, the invention provides for the use of a purified Fischer-Tropsch gasoil fraction produced by the process according to the present invention as a solvent, diluent or functional fluid.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides for a process for preparing purified Fischer-Tropsch gasoil fractions. These Fischer-Tropsch gasoil fractions are particularly suitable for use as solvents, diluents and functional fluids, in particular for the applications as mentioned herein.

In the process according to the invention, the fractions are prepared by providing and fractionating a Fischer-Tropsch-derived gasoil. Fischer-Tropsch-derived gasoil according to the present invention is a synthetic gasoil derived from a Fischer-Tropsch process. Fischer-Tropsch-derived gasoil is known in the art. By the term “Fischer-Tropsch-derived” is meant that the gasoil, is, or is derived from, a synthesis product of a Fischer-Tropsch process. In a Fischer-Tropsch process, synthesis gas is converted to a synthesis product. Synthesis gas or syngas is a mixture of predominantly hydrogen and carbon monoxide that is obtained by conversion of a hydrocarbonaceous feedstock. Suitable feedstocks include natural gas, crude oil, heavy oil fractions, coal, biomass or lignocellulosic biomass and lignite. A Fischer-Tropsch-derived gasoil may also be referred to as a GTL (Gas-to-Liquids) gasoil. The Fischer-Tropsch-derived gasoil is characterized as the product of a Fischer-Tropsch process wherein a synthesis gas, or mixture of predominantly hydrogen and carbon monoxide, is processed at elevated temperature over a supported catalyst comprised of a Group VIII metal, or metals, e.g., cobalt, ruthenium, iron, etc. At least part of the Fischer-Tropsch product is contacted with hydrogen, at hydrocracking/ hydroisomerization conditions over a, preferably, bifunctional, catalyst, or catalyst containing a metal, or metals, hydrogenation component and an acidic oxide support component active in producing both hydrocracking and hydroisomerization reactions. A least part of the resulting hydrocracked/hydroisomerized Fischer-Tropsch product may be provided as the Fischer-Tropsch-derived gasoil.

Fischer-Tropsch-derived gasoils are different from crude oil-derived gasoils. Despite having a similar boiling point range, the specific molecular composition of the Fischer-Tropsch-derived gasoils may allow for, amongst others, improved viscosity characteristics, improved pour point characteristics, improved density characteristics and in particular a combination of any of the aforementioned characteristics with specific desired flash point characteristics. For example, Fischer-Tropsch-derived gasoils may combine a low volatility with a high flash point, whereas the viscosity of such Fischer-Tropsch-derived gasoils may be lower than the viscosity of crude oil-derived gasoil having a similar volatility and flashpoint. Notwithstanding the above, the Fischer-Tropsch-derived gasoils is a complex mixture of molecules that should not be compared to a pure paraffinic molecule like e.g. pure n-dodecane.

The different characteristics of the Fischer-Tropsch-derived gasoils, compared to the crude oil-derived gasoils, are generally attributed to their particular isoparaffin to normal paraffin weight ratio (i/n ratio), relative amount of mono-methyl branched iso paraffins and the molecular weight distribution of the paraffins, as well as the absence of substantial levels aromatics and naphthenes.

A particular advantage of the Fischer-Tropsch-derived gasoils is that these gasoils exhibit very little odor and are almost colorless. Color as used herein is the Saybolt color as measured by its Saybolt number (ASTM D156: Standard Test Method for Saybolt Color of Petroleum Products). A high Saybolt number, +30, indicates colorless fluids, whereas lower Saybolt numbers, in particular below zero, indicate a discoloration. A Saybolt number lower than 25 already indicates the presence of a visually observable discoloration. Fischer-Tropsch-derived gasoils typically have the highest Saybolt number, i.e. +30. The high purity, low odor and minimal color characteristics, together with the above mentioned improved viscosity, pour point, density and flash point characteristics make the Fischer-Tropsch-derived gasoils highly suitable for solvent, diluent and functional fluid applications.

These low odor and minimal color characteristics are in part caused by the fact that, although contaminants may be present in the Fischer-Tropsch-derived gasoil feedstock, the concentration of these contaminants in the Fischer-Tropsch-derived gasoil is relatively low. This is due to the nature of the Fischer-Tropsch process for making the Fischer-Tropsch-derived gasoil, wherein the feedstock to the Fischer-Tropsch process comprises little to no sulphur and the process produces very little unsaturates, e.g. aromatics, oxygenates and nitrous compounds. It has now been found that it may be possible to meet specific requirements of particular applications of the Fischer-Tropsch-derived gasoil by fractionating the Fischer-Tropsch-derived gasoil into two or more fractions having a different boiling point range. By fractionating the Fischer-Tropsch-derived gasoil, isoparaffins and normal paraffins are distributed unevenly over the two or more fractions and Fischer-Tropsch gasoil fractions may be obtained that have an i/n ratio different from the Fischer-Tropsch-derived gasoil feedstock. Also the relative amount of mono-methyl branched isoparaffins and the molecular weight distribution of the paraffins may be different. As a consequence the viscosity, pour point, density and flash point characteristics of the Fischer-Tropsch-derived gasoil fractions may be change, beyond the change that would be expected on the basis of a fractionation based on boiling ranges alone.

In the process according to the present invention, a Fischer-Tropsch-derived gasoil feedstock is provided to a fractionation zone. Reference herein to a fractionation zone is to one or more separation means for separating the Fischer-Tropsch-derived gasoil feedstock into two or more fractions having a different boiling point range. Examples of suitable separating means include, but are not limited to, distillation units. Preferably, the Fischer-Tropsch-derived gasoil feedstock is fractionated by distillation. The Fischer-Tropsch-derived gasoil feedstock may be fractionated in a single distillation column or in two or more distillation columns. It is preferred that the Fischer-Tropsch-derived gasoil feedstock is fractionated in one or more distillation columns. In the process according to the invention, the Fischer-Tropsch-derived gasoil feedstock is preferably fractionated by distillation, wherein the Fischer-Tropsch-derived gasoil feedstock is fractionated, i.e. distilled, by heating part or all of the Fischer-Tropsch-derived gasoil feedstock to temperatures in the range of from 150 to 500° C.

In the fractionation zone, the Fischer-Tropsch-derived gasoil feedstock is fractionated into two or more Fischer-Tropsch-derived gasoil fractions, herein further referred to as Fischer-Tropsch gasoil fractions, each having a different boiling point range. Preferably, the Fischer-Tropsch-derived gasoil feedstock is fractionated into three or more, more preferably four or more, Fischer-Tropsch gasoil fractions each having a different boiling point range. Preferably at least one, more preferably at least two, of the Fischer-Tropsch gasoil fractions has a final boiling point of at most 260° C., preferably at most 250° C., more preferably at most 215° C. Reference herein to the final boiling point is to the higher limit of the boiling point range of a Fischer-Tropsch gasoil fraction, this boiling point range is defined as the range between the initial boiling point and final boiling point measured under atmospheric conditions as determined according to ASTM D86.

Equally preferable, at least one, more preferably at least two, of the Fischer-Tropsch gasoil fractions has an initial boiling point of more than 260° C., preferably at least 300° C., more preferably at least 310° C. Reference herein to the an initial boiling point is to the lower limit of the boiling point range of a Fischer-Tropsch gasoil fraction, this boiling point range is defined as the range between the initial boiling point and final boiling point measured under atmospheric conditions as determined according to ASTM D86.

Although, the Fischer-Tropsch-derived gasoil feedstock is virtually odorless and colorless, as mentioned herein before, the Fischer-Tropsch fractions obtained by fractionating the Fischer-Tropsch-derived gasoil feedstock may not all be odorless and/or colorless. When fractionating the Fischer-Tropsch-derived gasoil, the hydrocarbon compounds in the in the Fischer-Tropsch-derived gasoil feedstock are exposed to elevated temperatures, optionally in the presence of oxygen or oxygen-comprising compounds. As a result undesired side-reactions may take place during fractionation that form contaminants which were not, or not or albeit at a lower concentration, present in the Fischer-Tropsch-derived gasoil feedstock.

One particular side-reaction that may take place is the thermal dehydrogenation of one or more of the hydrocarbon compounds, in particular paraffins, in the Fischer-Tropsch-derived gasoil feedstock to unsaturated and/or multiple-unsaturated hydrocarbon compounds, including unsaturated aliphatic hydrocarbon compounds and aromatics. The thermal dehydrogenation of one or more of the hydrocarbon compounds may lead to a wide range of further or subsequent side-reactions, including but not limited to oxidation, aromatization, isomerization, oligomerization, cracking, and alkylation side reactions. In particular, oxidation and, to a more limited extent, aromatization and/or dehydro-aromatization reactions of unsaturated aliphatic hydrocarbon compounds may result in the formation of oxygenates and aromatic contaminants as by-products. The presence of metal surfaces in fractionation units, in particular distillation units, may catalyze the dehydrogenation and subsequent oxidation and aromatization and/or dehydro-aromatization reactions.

In particular the subsequent thermal oxidation of any formed unsaturated aliphatic hydrocarbon compounds to oxygenates contaminants may occur. In particular at temperatures above 180° C., the unsaturated aliphatic hydrocarbon compounds become increasingly more prone to thermal oxidation. Also the presence of metal surfaces in fractionation units, in particular distillation units, may catalyze the oxidation reactions. Oxidation herein refers to reactions with oxygen during the fractionation. The oxygen may be present in the fractionation unit. Oxygen may be introduced in the process in the form of air that is present in the fractionation units.

In addition to the above mentioned side reactions, the thermal oxidation of other hydrocarbon compounds in the Fischer-Tropsch-derived gasoil feedstock may occur.

In one particular case, the Fischer-Tropsch-derived gasoil feedstock provided in step (a) already contains one or more contaminants, in particular the Fischer-Tropsch-derived gasoil feedstock may contain one or more unsaturated aliphatic hydrocarbon compounds as contaminants. Although, the concentration of unsaturated aliphatic hydrocarbon compound contaminants in the Fischer-Tropsch-derived gasoil feedstock is small compared to crude oil-derived gasoils, these unsaturated aliphatic hydrocarbon compounds may nonetheless be oxidized to further oxygenates contaminants.

Paraffins in the Fischer-Tropsch-derived gasoil feedstock may be oxidized directly to oxygenates.

Contaminants herein are non-paraffinic, non naphthenic compounds. The term contaminant herein refers to compounds selected from the group consisting of oxygenate compounds and unsaturated hydrocarbon compounds.

The term oxygenate compounds herein refers to oxygen-containing hydrocarbonaceous compounds. Examples of oxygenates include, but are not limited to alcohols, ketones, aldehydes, ethers, epoxides and acids.

The term unsaturated hydrocarbon compounds herein refers to compounds having one or more unsaturated bonds, including unsaturated aliphatic hydrocarbon compounds and aromatics.

The term aromatics herein refers to aromatic compounds and compounds that comprise one or more aromatic groups, including polycyclic aromatics.

Oxygenate compounds and aromatics compounds are believed to provide the most significant contribution to the odor and discoloration of the contaminant-enriched Fischer-Tropsch gasoil fractions. Herein oxygenate compounds are believed to provide the most significant contribution to the odor and, to a lesser extent, discoloration of the contaminant-enriched Fischer-Tropsch gasoil fractions. Aromatics compounds are believed to provide the most significant contribution to the discoloration and odor of the contaminant-enriched Fischer-Tropsch gasoil fractions.

The term contaminant concentration herein refers to a contaminant concentration expressed in ppmw, unless expressly mentioned differently, and calculated on the basis of the respective total Fischer-Tropsch-derived gasoil, contaminant-enriched Fischer-Tropsch gasoil fraction, or purified Fischer-Tropsch gasoil fraction and the total weight of the contaminants.

The oxygenate compounds and aromatics compounds formed during the fractionation may be distributed differently over the two or more Fischer-Tropsch gasoil fractions. It has been observed that in particular the lower molecular weight and/or more volatile contaminants may accumulate in the lower boiling fractions rather than residing in the higher boiling fractions. These contaminants are believed to cause an undesired odor, and optionally a discoloration of the fraction.

It has been observed that in particular the higher molecular weight and/or less volatile contaminants may accumulate in the higher boiling fractions rather than residing in the lower boiling fractions. These contaminants are believed to predominantly cause an undesired discoloration of the fraction.

In the process according to the invention, at least one Fischer-Tropsch gasoil fraction prepared in step (a) is a contaminant-enriched Fischer-Tropsch gasoil fraction. Reference herein to a contaminant-enriched Fischer-Tropsch gasoil fraction is to a Fischer-Tropsch gasoil fraction that contains a higher concentration of one or more contaminants compared to the Fischer-Tropsch-derived gasoil feedstock. This includes contaminants formed during the fractionation that were not present in the Fischer-Tropsch-derived gasoil feedstock at all. In particular, the contaminant-enriched Fischer-Tropsch gasoil fraction may comprise at least one contaminant selected from the group consisting of oxygenate compounds and aromatics.

In the process according to the invention at least part of the contaminants in the contaminant-enriched Fischer-Tropsch fraction are formed during the fractionation of step (a). At least part of the contaminants in the contaminant-enriched Fischer-Tropsch fraction that were not formed during the fractionation of step (a) were preferably part of the Fischer-Tropsch-derived gasoil feedstock.

An effect of the formation of contaminants during the fractionation and consequently a presence or increased concentration of contaminants in the contaminant-enriched Fischer-Tropsch gasoil fraction may be that the Saybolt number of the contaminant-enriched Fischer-Tropsch gasoil fraction becomes undesirably low, or at least becomes lower than the Saybolt number of the Fischer-Tropsch-derived gasoil feedstock, i.e. the coloring of the fraction has increased. This undesired discoloration is particularly observed in the higher boiling Fischer-Tropsch gasoil fractions. Without wishing to be bound to any particular theory, it is believed that in particular more complex and conjugated molecules influence the emission and adsorption of lights. Such complex and conjugated molecules are more likely be of a higher molecular weight. It has been observed that discoloration of the lower fractions may also happen, albeit to a lesser extent.

A further effect of the formation of contaminants during the fractionation and consequently a presence or increased concentration of contaminants in the contaminant-enriched Fischer-Tropsch gasoil fraction may be an undesirable odor, or at least an undesirable increase of the odor emitted by the Fischer-Tropsch gasoil fraction compared to the Fischer-Tropsch-derived gasoil feedstock. This undesired odor is particularly observed in the lower boiling Fischer-Tropsch gasoil fractions. Without wishing to be bound to any particular theory, it is believed that in particular more volatile lower molecular weight molecules cause the existence of an odor.

Fischer-Tropsch-derived gasoils and in particular Fischer-Tropsch-derived paraffins intrinsically have an odor. Therefore, when herein reference is made to terms like odorless, low odor or little odor, reference is made to an odor that is qualitatively equal to or qualitatively similar to that of the Fischer-Tropsch-derived gasoils or paraffins. Where herein reference is made to terms like an increased odor, a stronger odor, and an undesired odor or similar qualifications, reference is made to an odor that is qualitatively different from to that of the Fischer-Tropsch-derived gasoils or paraffins. This difference in odor can be characterized by comparing the odor of a contaminant-enriched fraction to the Fischer-Tropsch-derived gasoil feedstock and classifying the difference, i.e. 1 is qualitatively equal to Fischer-Tropsch-derived gasoil feedstock (good odor characteristic), while 5 is qualitatively very different to Fischer-Tropsch-derived gasoil feedstock (poor odor characteristic).

Preferably, the Fischer-Tropsch-derived gasoil feedstock is fractionated into at least two Fischer-Tropsch gasoil fractions, of which at least two Fischer-Tropsch gasoil fractions at least one is a contaminant-enriched Fischer-Tropsch gasoil fraction having a final boiling point of at most 260° C., preferably at most 250° C., more preferably at most 215° C. Equally preferably, the Fischer-Tropsch-derived gasoil feedstock is fractionated into least two Fischer-Tropsch gasoil fractions, of which at least two Fischer-Tropsch gasoil fractions at least one is a at least one contaminant-enriched Fischer-Tropsch gasoil fraction having an initial boiling point of more than 260° C., preferably at least 300° C., more preferably at least 310° C. Typically, the Saybolt number of the contaminant-enriched Fischer-Tropsch gasoil fraction having an initial boiling point of more than 260° C. may be below 30, in particular at least 2, optionally at least 5, lower than the Saybolt number of the Fischer-Tropsch-derived gasoil feedstock.

Where more than one contaminant-enriched Fischer-Tropsch gasoil fraction is produced, the above described properties of the contaminant-enriched Fischer-Tropsch gasoil fraction may apply to at least one contaminant-enriched Fischer-Tropsch gasoil fraction, however, may apply to other contaminant-enriched Fischer-Tropsch gasoil fractions.

Both the above described discoloration and increase of odor are undesired properties and do not benefit the use of the Fischer-Tropsch gasoil fractions in solvent, diluent or functional fluid applications. To make the Fischer-Tropsch gasoil fractions suitable for a wider range of solvent, diluent or functional fluid applications, the contaminant-enriched Fischer-Tropsch should be further treated to reduce the odor and/or discoloration.

Therefore, the process according to the invention further includes providing the contaminant-enriched Fischer-Tropsch gasoil fraction to an absorption zone. The absorption zone comprises at least one absorbent material, which is suitable to absorb at least part of the contaminants. Reference herein to an absorbent material is to absorbent materials and adsorbent materials. Reference herein to absorb is to absorb and adsorb. Reference herein to absorption is to absorption and adsorption.

Preferably, the absorption zone comprises at least one absorbent material selected from the group consisting of magnesium silicate and molecular sieve materials including 4A or 5A molecular sieve, zeolite X, zeolite 13X, zeolite Y, dealuminized zeolite Y, ultrastable Y, ZSM-12, mordenite, zeolite beta, zeolite L, zeolite omega.

Without wishing to be bound by any particular theory, it is believed that in particular absorbent materials having larger pore sizes, i.e. above 0.5 nm (5 Angstrom) or larger, are able to absorb the relative large aromatic compounds in addition to the oxygenates and other contaminants. Absorbent materials having smaller pore sizes absorb predominately the non-aromatic compounds, including in particular oxygenates.

Therefore, it is particularly preferred that the absorption zone comprises at least one absorbent material comprising pores having a pore size of more than 0.5 nm (5 Angstrom), more preferably at least 0.55 nm (5.5 Angstrom), even more preferably at least 0.6 nm (6 Angstrom), still more preferably at least 0.65 nm (6.5 Angstrom). Preferably, the absorption zone comprises at least one absorbent material that is a molecular sieve material selected from the group consisting of, zeolite X, zeolite 13X, zeolite Y, dealuminized zeolite Y, ultrastable Y, ZSM-12, mordenite, zeolite beta, zeolite L, zeolite omega, more preferably zeolite X, zeolite 13X, zeolite Y, dealuminized zeolite Y, ultrastable Y, ZSM-12, mordenite, zeolite beta, zeolite L, zeolite omega, even more preferably the absorbent material is zeolite 13X, which is the sodium form of zeolite X. Where the absorption zone comprises at least one molecular sieve absorbent material is preferred that at least one molecular sieve absorbent material has a channel structure in one or more directions having a diameter of more than 0.5 nm (5 Angstrom), more preferably at least 0.55 nm (5.5 Angstrom), even more preferably at least 0.6 nm (6 Angstrom), still more preferably at least 0.65 nm (6.5 Angstrom).

The absorption zone may comprise two or more of absorbent materials, preferably two or more selected from the absorbent materials described hereinabove. A preferred combination of absorbent material may include zeolite 13X and magnesium silicate. Another preferred combination of absorbent material may include zeolite 13X and active coal. The combination of absorbent may be able to more efficiently absorb a broader range of contaminants, for instance both larger and smaller molecular size contaminants, e.g. oxygenates and aromatics, or polar and non-polar contaminants.

The molecular sieve used as the absorbent material in the process of the invention is preferably based on an acidic molecular sieve having a framework silica-to-alumina molar ratio less than 100 and more preferably greater than 10, for example 20 to 50. Lower silica materials have more sites for available adsorption sites and therefore may be more effective than the higher silica molecular sieve materials.

The absorbent material used in the absorption zone of the process of the invention may be provided in the form of particles, for example extrudate, spheres or pellets. The particles may comprise the absorbent material alone or together with a binder material or filler material to improve the strength of the particles. The binder or filler material may be for instance amorphous metal oxides, including alumina, silica, zirconia, and titania. Preferably the binder or filler material is alumina.

Preferably, the contaminant-enriched Fischer-Tropsch gasoil fraction is contacted with the absorbent material in the absorption zone at a temperature in the range of from 0 to 150° C. The lower limit of the temperature range at which the contaminant-enriched Fischer-Tropsch gasoil fraction is contacted with the absorbent material in the absorption zone is related to the fact that the absorption is diffusion limited and temperatures below 0° C. would cause an undesirable decrease of the diffusion rate of the contaminants from the contaminant-enriched Fischer-Tropsch gasoil fraction to the absorbent material. When the contact temperature is increased, i.e. above 0° C., the rate of diffusion may increase. By maintaining the temperature below 150° C., the formation of by-products is reduced. This is important as these by-products may have an undesirable effect on the applicability of the resulting Fischer-Tropsch gasoil fraction.

More preferably, the contaminant-enriched Fischer-Tropsch gasoil fraction is contacted with the absorbent material in the absorption zone at a temperature in the range of from 10 to 40° C., most preferably in the range of from 10 to 30° C.

Preferably, the contaminant-enriched Fischer-Tropsch gasoil fraction is contacted with the absorbent material in the absorption zone at a pressure in the range of from 1 to 75 bar, preferably of from 1.1 to 50 bar.

The contaminant-enriched Fischer-Tropsch gasoil fraction may contacted with the absorbent material batch-wise or in a continuous mode. It is preferred that the contaminant-enriched Fischer-Tropsch gasoil fraction is contacted with the absorbent material under turbulent flow conditions to stimulate the fluid/solid material interaction. In the continuous mode case, the absorption zone may preferably comprise a fixed bed reactor comprising at least one fixed bed of absorbent material.

Preferably, the contaminant-enriched Fischer-Tropsch gasoil fraction is contacted with the absorbent material under continuous stirring or induced mixing, this is particularly preferred in case of a batch-wise operation.

Preferably, the contaminant-enriched Fischer-Tropsch gasoil fraction is contacted with the absorbent material in the absorption zone at for a time sufficient to absorb at least part of the contaminants. In case of a batch-wise contacting of the contaminant-enriched Fischer-Tropsch gasoil fraction with the absorbent material, the contaminant-enriched Fischer-Tropsch gasoil fraction may be contacted with the absorbent material for any time in the range of from 1 minute to 48 hours, preferably for of from 30 minutes to 24 hours, more preferably of from 60 minutes to 24 hours. Preferably, in a batch contacting, the contaminant-enriched Fischer-Tropsch gasoil fraction may be contacted with the absorbent material in a volume ratio of contaminant-enriched Fischer-Tropsch gasoil fraction over absorbent material of in the range of from 0.5 to 200, more preferably of from 1 to 175, even more preferably 5 to 125.

In case of a continuous contacting of the contaminant-enriched Fischer-Tropsch gasoil fraction with the absorbent material, the contaminant-enriched Fischer-Tropsch gasoil fraction may be contacted with the absorbent material for any time in the range of from 1 minute to 48 hours, preferably for of from 30 minutes to 24 hours, more preferably of from 60 minutes to 24 hours. Preferably, the contaminant-enriched Fischer-Tropsch gasoil fraction is contacted with the absorbent material in the absorption zone at a LHSV of from 0.0001 to 0.01 s^(−1,) more preferably 0.0001 to 0.005 s^(−s), still more preferably 0.0001 to 0.003 s⁻¹.

The absorption zone may comprise one or more absorption sections. In one embodiment, the absorption zone may comprise two or more absorption sections in-series. Optionally, the absorption zone may comprise two or more sections, each comprising a separate absorbent. This has the advantage that different contaminants may be separately removed, to the extent required. One example may be a first section comprising Mg silicate or similar absorbent material and a second section comprising zeolite 13X or similar large pore molecular sieve absorbent material. This combination has the advantage that the Mg silicate or similar absorbent material can absorb part of the oxygenates, allowing a larger part of the absorption capacity of the zeolite 13X or similar large pore molecular sieve absorbent material to be used for the aromatic contaminants. Alternatively, the absorption zone may comprise a mixture of two or more absorbent materials.

In a further embodiment, the absorption zone may comprise two or more absorption sections in parallel, preferably comprising the same absorbent material. An advantage of providing parallel absorption zones is that this allows for a continuous operation of the absorption process, wherein alternating absorption beds are regenerated (as described in more detail herein below), while the remaining section are in a normal operation mode.

Other embodiments may include both parallel as well as in-series aligned absorption zones.

In the process according to the invention, a purified Fischer-Tropsch gasoil fraction is retrieved from the absorption zone as the high purity Fischer-Tropsch gasoil fraction. The purified Fischer-Tropsch gasoil fraction retrieved from the absorption zone is contaminant-depleted, i.e. the purified Fischer-Tropsch gasoil fraction comprises a contaminant concentration that is lower than the contaminant concentration of the contaminant-enriched Fischer-Tropsch gasoil fraction. Preferably, at least one of the aromatic and oxygenate concentrations of the purified Fischer-Tropsch gasoil fraction is lower the corresponding concentrations of the contaminant-enriched Fischer-Tropsch gasoil fraction. More preferably, the aromatic and oxygenate concentrations of the purified Fischer-Tropsch gasoil fraction are lower the corresponding concentrations of the contaminant-enriched Fischer-Tropsch gasoil fraction.

Preferably, the purified Fischer-Tropsch gasoil fraction according to the present invention comprises:

-   -   in the range of from 0 to 500 ppmw, more preferably of from 0 to         200 ppmw, still more preferably of from 0 to 100 ppmw, even         still more preferably of from 0 to 50 ppmw, most preferably of         from 0 to 25 ppmw of aromatics, based on the weight of the         purified Fischer-Tropsch gasoil fraction;     -   in the range of from 0 to 3 ppmw, more preferably of from 1         ppmw, of oxygenates, calculated based on the weight of the         elemental oxygen in the oxygenates and the weight of the         purified Fischer-Tropsch gasoil fraction; and/or     -   an unsaturated aliphatic hydrocarbon compound concentration         corresponding to a Br index (mg Br/100g feedstock) of in the         range of from 0 to 100, preferably of from 0 to 75, more         preferably of from 0 to 50, still more preferably of from 0 to         35 as measured according to ASTM D2710.

More preferably, the purified Fischer-Tropsch gasoil fraction according to the present invention comprises:

-   -   in the range of from 0 to 500 ppmw, more preferably of from 0 to         200 ppmw, still more preferably of from 0 to 100 ppmw, even         still more preferably of from 0 to 50 ppmw, most preferably of         from 0 to 25 ppmw of aromatics, based on the weight of the         purified Fischer-Tropsch gasoil fraction; and     -   in the range of from 0 to 3 ppmw, more preferably of from 1         ppmw, of oxygenates, calculated based on the weight of the         elemental oxygen in the oxygenates and the weight of the         purified Fischer-Tropsch gasoil fraction. Herein above reference         is made to elemental oxygen to indicate that the concentration         of oxygenates is determined on the basis of the weight of the         oxygen atoms present in the gasoil and not on the weight to the         complete molecule comprising those oxygen atoms.

The contaminants, if any, remaining in the purified Fischer-Tropsch gasoil fraction may have been formed during the fractionation of the Fischer-Tropsch-derived gasoil feedstock or may have been present in the Fischer-Tropsch-derived gasoil feedstock prior to fractionation.

In case there is more than one contaminant-enriched Fischer-Tropsch gasoil fraction, each contaminant-enriched Fischer-Tropsch gasoil fraction is preferably provided to separate absorption zones with separate absorption material(s) to prepare more than one purified Fischer-Tropsch gasoil fractions. This is particularly relevant when the fractionation and absorption process steps are operated in a continuous mode. Each of such individual more than one purified Fischer-Tropsch gasoil fractions may preferably have its own specific contaminant concentration within the above mentioned ranges.

It is an advantage of the process according to the invention that nitrogen-containing compounds and/or sulfur-containing compounds introduced during the fractionation may also be absorbed in addition to the oxygenates and aromatics.

In addition to the purified Fischer-Tropsch gasoil fraction, contaminant-comprising absorbent material may be retrieved from the absorption zone. The contaminant-comprising absorbent material may be recycled to the absorption zone or, in particular when the sorption capacity of the absorbent material is reached, the contaminant-comprising absorbent material may be regenerated. The absorbent material may be regenerated in any suitable manner that will desorb or otherwise remove the contaminants from the absorbent material. For instance, the absorbent material may be regenerated either by stripping with a desorbent, such as steam or nitrogen, or by heating the absorbent material in the presence of for example oxygen, oxygen enriched air, air or a hydrogen-containing gas to burn off, or otherwise decompose the absorbed contaminants. After the absorbent material has been regenerated it may be recycled to the absorption zone.

In addition to contaminant-enriched Fischer-Tropsch gasoil fractions, also Fischer-Tropsch gasoline fractions that are not contaminant-enriched may be prepared in step a). Such Fischer-Tropsch gasoline fractions that are not contaminant-enriched are Fischer-Tropsch gasoline fractions that can be directly retrieved from the process, i.e. without subjecting these fractions to the absorption step (b) as further Fischer-Tropsch gasoline fractions.

The purified Fischer-Tropsch gasoline fraction retrieved from the absorption zone as the purified Fischer-Tropsch gasoline fraction (step (c)) may be used, optionally after even further treatment, for a desired application.

In a further aspect, the present invention provides the use of the purified Fischer-Tropsch gasoil fraction as a solvent, in a functional fluid formulation or a diluent. A functional fluid formulation herein may be a formulation comprising the purified Fischer-Tropsch gasoil fraction, preferably comprising further containing an additive compound. Typically, the solvents, functional fluid formulations and diluents may be used in many areas, for instance oil and gas exploration and production, process oils, agro chemicals, process chemicals, construction industry, food and related industries, paper, textile and leather, and various household and consumer products. Further, the type of additives used in the functional fluid formulation according to the present invention is dependent on the type of fluid formulation. Additives for functional fluid formulations include, but are not limited to, corrosion and rheology control products, emulsifiers and wetting agents, borehole stabilizers, high pressure and anti-wear additives, de- and anti-foaming agents, pour point depressants, and antioxidants.

Preferred solvent, diluent and/or functional fluid applications using the purified Fischer-Tropsch gasoil fraction obtained in the process according to the present invention as diluent oil or base oil include, but is not limited to, drilling fluids, heating fuels, lamp oil, barbeque lighters, concrete demoulding, pesticide spray oils, paints and coatings, personal care and cosmetics, consumer goods, pharmaceuticals, industrial and institutional cleaning, adhesives, inks, air fresheners, sealants, explosives, water treatment, cleaners, polishes, car dewaxers, electric discharge machining, transformer oils, process oil, process chemicals, silicone mastic, two stroke motor cycle oil, metal cleaning, dry cleaning, lubricants, metal work fluid, aluminum roll oil, explosives, chlorinated paraffins, heat setting printing inks, Timber treatment, polymer processing oils, rust prevention oils, shock absorbers, greenhouse fuels, fracturing fluids and fuel additives formulations.

Typical solvent, diluent and functional fluid applications are for example described in “The Index of Solvents”, Michael Ash, Irene Ash, Gower publishing Ltd, 1996, ISBN 0-566-07884-8 and in “Handbook of Solvents”, George Wypych, Willem Andrew publishing, 2001, ISBN 0-8155-1458-1.

An advantage of the use of the purified Fischer-Tropsch gasoil fraction as solvents, diluents or in functional fluid formulations is that the purified Fischer-Tropsch gasoil fraction has a low viscosity, low pour point while having a high flash point. This combination of physical characteristics of the purified Fischer-Tropsch gasoil fraction is highly desirable for its use in functional fluid formulations with low viscosity requirements.

For example, in drilling fluid applications, during use, the temperature of the drilling fluid may decrease which may lead to an increase of the viscosity of the drilling fluid. The high viscosity may be harmful for the beneficial use of the drilling fluid. Therefore, a purified Fischer-Tropsch gasoil fraction obtained from the process according to the present invention with a low viscosity and high flash point is highly desirable for its use in drilling fluid applications.

The use of the purified Fischer-Tropsch gasoil fraction as a diluent may include the use as a diluent oil or base oil for solvent and/or functional fluid applications.

With the term diluent oil is meant an oil used to decrease viscosity and/or improve other properties of solvent and functional fluid formulations.

With the term base oil is meant an oil to which other oils, solvents or substances are added to produce a solvent or functional fluid formulation.

The advantages of the use of the purified Fischer-Tropsch gasoil fraction as a diluent oil or base oil for solvent and/or functional fluid formulations are the same as described above for functional fluid formulations comprising the purified Fischer-Tropsch gasoil fraction, further containing an additive compound.

In a further aspect, the present invention provides the use of a purified Fischer-Tropsch gasoil fraction obtained by the process according to the invention for improving biodegradability and lower toxicity in solvent and/or functional fluid applications.

As described above, the purified Fischer-Tropsch gasoil fraction has preferably very low levels of aromatics, sulphur, nitrogen compounds and is preferably free from polycyclic aromatic hydrocarbons. These low levels may lead to, but are not limited to, low aquatic toxicity, low sediment organism toxicity and low terrestrial ecotoxicity of the purified Fischer-Tropsch gasoil fraction. The molecular structure of the purified Fischer-Tropsch gasoil fraction may lead to the readily biodegradability of the Fischer-Tropsch-derived gasoil.

Any further Fischer-Tropsch gasoil fraction obtained in step (a) of the process that is not contaminant-enriched also may be used in one or more ways as described herein above, similar to the purified Fischer-Tropsch gasoil fraction.

The specific use a particular Fischer-Tropsch gasoil fraction may be dependent on the exact composition and characteristics of that particular Fischer-Tropsch gasoil fraction. The Fischer-Tropsch-derived gasoil feedstock as provided in step (a) of the process according to the invention as feedstock is a synthetic gasoil derived from a feedstock other than crude oil, such a methane, coal or biomass, and produced in a Fischer-Tropsch process. The preparation of the Fischer-Tropsch-derived gasoils has been described in e.g. WO02/070628 and WO-A-9934917 (in particular the process as described in Example VII of WO-A-9934917, using the catalyst of Example III of WO-A-9934917), both of which are hereby incorporated by reference. As mentioned above these Fischer-Tropsch-derived gasoils have a different molecular composition and have significantly different properties compared to crude oil-derived gasoil. Therefore, Fischer-Tropsch-derived gasoils can be clearly distinguished from crude oil-derived gasoils. A number of preferred properties of the Fischer-Tropsch-derived gasoils are provided herein.

Preferably, the Fischer-Tropsch-derived gasoil comprises more than 50 wt % of iso-paraffins, preferably more than 70 wt % of iso-paraffins, still more preferably more than 80 wt %. Preferably, Fischer-Tropsch-derived gasoils have a i/n ratio of at least 2, more preferably at least 2.8, even more preferably at least 3.5, still more preferably at least 3.7, even still more preferably at least 4, yet more preferably at least 4.5.

Preferably, the Fischer-Tropsch-derived gasoil comprises in the range of from 20 to 40 wt %, preferably of from 21 to 37 wt %, more preferably of from 23 to 37 wt % of mono-methyl branched isoparaffins, based on the total weight of isoparaffins in the Fischer-Tropsch-derived gasoil.

Preferably, the Fischer-Tropsch-derived gasoil has an initial boiling point of at least 150° C. and a final boiling point of at most 450° C. at atmospheric conditions. Suitably, the Fischer-Tropsch-derived gasoil has an initial boiling point of at least 175° C. at atmospheric conditions, as determined using ASTM D86. It is noted herein that the initial boiling points, final boiling points and boiling ranges provided herein when describing the present invention are those initial boiling points, final boiling points and boiling ranges as determined by ASTM D86. It is further noted that the initial boiling points, final boiling points and boiling ranges as determined by ASTM D86 for the whole of the Fischer-Tropsch-derived gasoil do not exclude the presence of compounds or fractions that have a true boiling temperature below or above the respective ASTM D86-based initial boiling point and ASTM D86-based final boiling point of the whole of the Fischer-Tropsch-derived gasoil.

The Fischer-Tropsch-derived gasoil feedstock preferably has a final boiling point of from 330 to 450° C., more preferably of from 331 to 370° C., even more preferably of from 332 to 365° C. 333 to 351° C., still more preferably of from 336 to 348° C. and even still more preferably of from 339 to 345° C. at atmospheric conditions. By boiling points at atmospheric conditions is meant atmospheric boiling points, which boiling points are determined by ASTM D86.

The Fischer-Tropsch-derived gasoil, also referred to as Fischer-Tropsch full range gasoil, is a fluid comprising paraffins, including iso-paraffins and normal paraffins, with alkyl chain lengths in the range of from 7 to 30 carbon atoms, preferably comprising paraffins having from 9 to 25 carbon atoms; the Fischer-Tropsch-derived paraffin gasoil comprises preferably at least 70 wt %, more preferably at least 85 wt %, more preferably at least 90 wt %, more preferably at least 95 wt %, and even more preferably at least 98 wt % of Fischer-Tropsch-derived paraffins having 9 to 25 carbon atoms based on the total amount of Fischer-Tropsch-derived paraffins, preferably based on the amount of Fischer-Tropsch-derived paraffins having of from 7 to 30 carbon atoms.

Further, the Fischer-Tropsch-derived gasoil preferably has a density at 15° C. according to ASTM D4052 of from 774 kg/m³ to 782 kg/m³, more preferably of from 775 kg/m³ to 780 kg/m³, and even more preferably of from 776 kg/m³ to 779 kg/m³.

Suitably, the kinematic viscosity at 40° C. according to ASTM D445 of the Fischer-Tropsch-derived gasoil is of from 2.3 to 3.0 cSt, preferably of from 2.5 cSt to 2.9 cSt.

Further, the pour point of the Fischer-Tropsch-derived gasoil (according to ASTM D97) is preferably below −10° C., more preferably below −15° C., more preferably below −17° C., more preferably below −20° C., more preferably below −22° C., and even more preferably below −27° C. and preferably above −40° C.

Suitably, the cloud point of the Fischer-Tropsch-derived gasoil (according to ASTM D2500) is preferably below −10° C., more preferably below −15° C., more preferably below −18° C., more preferably below −20° C., more preferably below −22° C., and most preferably below −27° C. and preferably above −40° C.

Preferably, the flash point of the Fischer-Tropsch-derived gasoil according to ASTM D93 is of at least 60° C., more preferably 70° C., even more preferably at least 80° C. and still more preferably at least 85° C.

The Fischer-Tropsch-derived gasoil has a smoke point according to ASTM D1322 of more than 50 mm.

Typically, the Fischer-Tropsch-derived gasoil provided as feedstock to the process according to the present invention comprises:

-   -   in the range of from 0 to 300 ppmw, more preferably of from 0 to         200 ppmw, still more preferably of from 0 to 100 ppmw, even         still more preferably of from 0 to 50 ppmw, most preferably of         from 0 to 25 ppmw of aromatics, based on the weight of the         Fischer-Tropsch-derived gas oil;     -   in the range of from 0 to 3 ppmw, more preferably of from 0 to 1         ppmw, of oxygenates, calculated based on the weight of the         elemental oxygen in the oxygenates and the weight of the         Fischer-Tropsch-derived gasoil;     -   an unsaturated aliphatic hydrocarbon compound concentration         corresponding to a Br index (mg Br/100g feedstock) of in the         range of from 0 to 100, preferably of from 0 to 75, more         preferably of from 0 to 50, still more preferably of from 0 to         35 as measured according to ASTM D2710;     -   in the range of from 0 to 3 ppmw, more preferably of from 0 to 1         ppmw, still more preferably of from 0 to 0.2 ppmw, of         sulphur-containing hydrocarbonaceous compounds, calculated based         on the weight of the elemental sulphur in the sulphur-containing         hydrocarbonaceous compounds and the weight of the         Fischer-Tropsch-derived gasoil;     -   in the range of from 0 to 1 ppmw of nitrogen-containing         hydrocarbonaceous compounds, calculated based on the weight of         the elemental nitrogen in the nitrogen-containing         hydrocarbonaceous compounds and the weight of the         Fischer-Tropsch-derived gasoil; and/or     -   in the range of from 0 to 2 wt % naphthenics, based on the         weight of the Fischer-Tropsch-derived gasoil, wherein preferably         at least one of aromatics, oxygenates, sulphur-containing         hydrocarbonaceous compounds and nitrogen-containing         hydrocarbonaceous compounds is contained in the         Fischer-Tropsch-derived gasoil feedstock, i.e. at least one of         the above concentrations is not zero. Particularly, at least one         of aromatics, unsaturated aliphatic hydrocarbon compound and         oxygenates is contained in the Fischer-Tropsch-derived gasoil         feedstock, i.e. at least one of the above aromatic, unsaturated         aliphatic hydrocarbon compound and oxygenate concentration is         not zero.

Preferably, the Fischer-Tropsch-derived gasoil provided as feedstock to the process according to according to the present invention comprises:

-   -   in the range of from 0 to 300 ppmw, more preferably of from 0 to         200 ppmw, still more preferably of from 0 to 100 ppmw, even         still more preferably of from 0 to 50 ppmw, most preferably of         from 0 to 25ppmw of aromatics, based on the weight of the         Fischer-Tropsch-derived gas oil;     -   in the range of from 0 to 3 ppmw, more preferably of from 0 to 1         ppmw, of oxygenates, calculated based on the weight of the         elemental oxygen in the oxygenates and the weight of the         Fischer-Tropsch-derived gasoil;     -   an unsaturated aliphatic hydrocarbon compound concentration         corresponding to a Br index (mg Br/100 g feedstock) of in the         range of from 0 to 100, preferably of from 0 to 75, more         preferably of from 0 to 50, still more preferably of from 0 to         35 as measured according to ASTM D2710;     -   in the range of from 0 to 3 ppmw, more preferably of from 0 to 1         ppmw, still more preferably of from 0 to 0.2 ppmw, of         sulphur-containing hydrocarbonaceous compounds, calculated based         on the weight of the elemental sulphur in the sulphur-containing         hydrocarbonaceous compounds and the weight of the         Fischer-Tropsch-derived gasoil;     -   in the range of from 0 to 1 ppmw of nitrogen-containing         hydrocarbonaceous compounds, calculated based on the weight of         the elemental nitrogen in the nitrogen-containing         hydrocarbonaceous compounds and the weight of the         Fischer-Tropsch-derived gasoil; and     -   in the range of from 0 to 2 wt % naphthenics, based on the         weight of the Fischer-Tropsch-derived gasoil, wherein preferably         at least one of aromatics, unsaturated aliphatic hydrocarbon         compounds, oxygenates, sulphur-containing hydrocarbonaceous         compounds and nitrogen-containing hydrocarbonaceous compounds is         contained in the Fischer-Tropsch-derived gasoil feedstock, i.e.         at least one of the above concentrations is not zero.         Particularly, at least one of aromatics, unsaturated aliphatic         hydrocarbon compound and oxygenates is contained in the         Fischer-Tropsch-derived gasoil feedstock, i.e. at least one of         the above aromatic, unsaturated aliphatic hydrocarbon compounds,         and oxygenates concentration is not zero. Herein above reference         is made to elemental oxygen, elemental sulphur and elemental         nitrogen to indicate that the concentration of oxygenates,         sulphur-containing hydrocarbonaceous compounds and         nitrogen-containing hydrocarbonaceous compounds is determined on         the basis of the weight of the oxygen, sulphur and nitrogen         atoms present in the gasoil and not on the weight to the         complete molecule comprising those oxygen, sulphur and nitrogen         atoms.

Further, the Fischer-Tropsch-derived gasoil preferably comprises less than 300 ppmw polycyclic aromatic hydrocarbons, more preferably less than 25 ppmw polycyclic aromatic hydrocarbons and most preferably less than 1 ppmw polycyclic aromatic hydrocarbons, based on the weight of the Fischer-Tropsch-derived gasoil. Further, the Fischer-Tropsch-derived gasoil comprises n-paraffins and may comprise cyclo-alkanes.

It is an advantage of the process according to the invention that undesired compounds already included in the feedstock may also be absorbed in step (b) in addition to the oxygenates and aromatics formed during the fractionation in step (a).

In step (a) of the process according to the invention, the Fischer-Tropsch-derived gasoil feedstock is fractionated into two or more Fischer-Tropsch gasoil fractions, preferably three or more Fischer-Tropsch gasoil fractions, more preferably four or more Fischer-Tropsch gasoil fractions. Particularly, at least one of the Fischer-Tropsch gasoil fractions, in particular a contaminant-enriched Fischer-Tropsch gasoil fraction, obtained in step (a) may exhibits a stronger odor than the Fischer-Tropsch-derived gasoil feedstock.

Preferably, at least one of the Fischer-Tropsch gasoil fractions obtained in step (a), or ultimately (c) if the Fischer-Tropsch gasoil fraction is provided to step (b), has a final boiling point of at most 260° C., preferably at most 250° C., more preferably at most 215° C. Preferably, at least one of the Fischer-Tropsch gasoil fractions is a Fischer-Tropsch gasoil fraction selected from the group consisting of (1) a Fischer-Tropsch gasoil fraction having a final boiling point of at most 180° C., preferably at most 170° C., (2) a Fischer-Tropsch gasoil fraction an initial boiling point of at least 160° C., preferably at least 170° C., and a final boiling point of at most 200° C., preferably at most 190° C., (3) a Fischer-Tropsch gasoil fraction an initial boiling point of at least 180° C., preferably at least 190° C., and a final boiling point of at most 225° C., preferably at most 215° C., and (4) a Fischer-Tropsch gasoil fraction an initial boiling point of at least 205° C., preferably at least 215° C., and a final boiling point of at most 260° C., preferably at most 250° C., wherein the boiling point is measured at atmospheric conditions, as determined using ASTM D86.

In particular, at least one of the Fischer-Tropsch gasoil fractions obtained in step (a), or ultimately (c) if the Fischer-Tropsch gasoil fraction is provided to step (b), may be a Fischer-Tropsch gasoil fraction selected from the group consisting of (1) a Fischer-Tropsch gasoil fraction a final boiling point of at most 180° C., preferably at most 170° C., (2) a Fischer-Tropsch gasoil fraction an initial boiling point of at least 160° C., preferably at least 170° C., and a final boiling point of at most 200° C., preferably at most 190° C., and (3) a Fischer-Tropsch gasoil fraction an initial boiling point of at least 180° C., preferably at least 190° C., and a final boiling point of at most 225° C., preferably at most 215° C., wherein the boiling point is measured at atmospheric conditions, as determined using ASTM D86.

More in particular, at least one of the Fischer-Tropsch gasoil fractions obtained in step (a), or ultimately (c) if the Fischer-Tropsch gasoil fraction is provided to step (b), may be a Fischer-Tropsch gasoil fraction selected from the group consisting of (2) a Fischer-Tropsch gasoil fraction an initial boiling point of at least 160° C., preferably at least 170° C., and a final boiling point of at most 200° C., preferably at most 190° C., (3) a Fischer-Tropsch gasoil fraction an initial boiling point of at least 180° C., preferably at least 190° C., and a final boiling point of at most 225° C., preferably at most 215° C., wherein the boiling point is measured at atmospheric conditions, as determined using ASTM D86.

It has been found that such Fischer-Tropsch gasoil fractions obtained in step (a) having a final boiling point of at most 260° C., and preferably lower, are especially prone to having undesired odor characteristics and therefore benefit from the absorption process in step (b) to remove contaminants that cause the undesired odor.

Preferably, at least one of the above described fractions obtained in step (a) is provided to step (b) as the contaminant-enriched Fischer-Tropsch gasoil fraction.

At least one of the Fischer-Tropsch gasoil fractions, in particular a contaminant-enriched Fischer-Tropsch gasoil fraction, obtained in step (a) may exhibit a color that results in a Saybolt number that is lower than the Saybolt number of the Fischer-Tropsch-derived gasoil feedstock.

The high purity Fischer-Tropsch gasoil fraction(s), retrieved as the purified Fischer-Tropsch gasoil fraction(s) in step (c), will have essentially the same boiling point ranges as the contaminant-enriched Fischer-Tropsch gasoil fraction provided to step (b) to prepare said purified Fischer-Tropsch gasoil fraction.

Preferably, at least one of the Fischer-Tropsch gasoil fractions obtained in step (a), or ultimately (c) if the Fischer-Tropsch gasoil fraction is provided to step (b), has an initial boiling point of more than 260° C., preferably at least 300° C., more preferably at least 310° C. Preferably, at least one of the Fischer-Tropsch gasoil fractions is a Fischer-Tropsch gasoil fraction selected from the group consisting of (1) a Fischer-Tropsch gasoil fraction an initial boiling point of more than 260° C., preferably at least 270° C. and a final boiling point of at most 320° C., preferably at most 310° C., (2) a Fischer-Tropsch gasoil fraction having an initial boiling point of at least 310° C., preferably at least 330° C., more preferably at least 360 ° C., wherein the boiling point is measured at atmospheric conditions, as determined using ASTM D86.

In particular, at least one of the Fischer-Tropsch gasoil fractions obtained in step (a), or ultimately (c) if the Fischer-Tropsch gasoil fraction is provided to step (b), may be a Fischer-Tropsch gasoil fraction having an initial boiling point of at least 310° C., preferably at least 330° C., more preferably at least 360° C., wherein the boiling point is measured at atmospheric conditions, as determined using ASTM D86.

It has been found that particularly the Fischer-Tropsch gasoil fractions obtained in step (a) and having an initial boiling point that is more than 260° C. are prone to discoloration, i.e. having a Saybolt number that is lower than the Fischer-Tropsch-derived gasoil feedstock, more in particular below having a Saybolt number below 30. Still more in particular is has been found that a contaminant-enriched Fischer-Tropsch gasoil fraction obtained in step (a) and having an initial boiling point that is more than 330° C., particularly more than 360° C., obtained in step (a) may have a Saybolt number that is lower than 28, in particular lower than 27, more in particular lower than 25.

Preferably, at least one of the above described fractions obtained in step (a) is provided to step (b) as the contaminant-enriched Fischer-Tropsch gasoil fraction.

The high purity Fischer-Tropsch gasoil fraction(s), retrieved as the purified Fischer-Tropsch gasoil fraction(s) in step (c), will have essentially the same boiling point ranges as the contaminant-enriched Fischer-Tropsch gasoil fraction provided to step (b) to prepare said purified Fischer-Tropsch gasoil fraction.

Preferably, at least one of the Fischer-Tropsch gasoil fractions obtained in step (a) or purified Fischer-Tropsch gasoil fraction(s) retrieved in step (c) as the purified Fischer-Tropsch gasoil fraction has an i/n ratio of in the range of from 2 to 6. Preferably, the majority, i.e. more than half, of the Fischer-Tropsch gasoil fractions obtained in step (a)or (c) has a has an i/n ratio of in the range of from 2 to 6. A high i/n ratio may advantageously effect on amongst others the viscosity of the Fischer-Tropsch gasoil fractions. Increasing the relative concentration of isoparaffin may lower the overall viscosity of the Fischer-Tropsch gasoil fractions. By fractionating the Fischer-Tropsch-derived gasoil feedstock, fraction may be obtained that have an improved i/n ratio, depending on the particular envisaged application.

Preferably, at least one of the Fischer-Tropsch gasoil fractions obtained in step (a) or (c) comprises in the range of from 30 to 75 wt %, more preferably of from 35 to 70wt %, more preferably of from 35 to 60wt % of mono-methyl branched isoparaffins, based on the total weight of isoparaffins in the Fischer-Tropsch gasoil fraction.

Preferably, the majority, i.e. more than half, of the Fischer-Tropsch gasoil fractions obtained in step (a) or (c) comprises in the range of from 30 to 75 wt % of mono-methyl branched isoparaffins, based on the total weight of isoparaffins in the Fischer-Tropsch gasoil fraction. Preferably, at least one of the Fischer-Tropsch gasoil fractions obtained in step (a) or (c) comprises a higher weight percentage of mono-methyl branched isoparaffins, based on the total weight of isoparaffins than the Fischer-Tropsch-derived gasoil feedstock. More preferably, at least two, still more preferably three, of the Fischer-Tropsch gasoil fractions obtained in step (a) or (c) comprises a higher weight percentage of mono-methyl branched isoparaffins, based on the total weight of isoparaffins than the Fischer-Tropsch-derived gasoil feedstock.

Mono-methyl branched isoparaffins exhibit desirable bio degradation characteristic compared to other isoparaffins. A relative high concentration of mono-methyl isoparaffins to other isoparaffins may advantageously effect amongst others the bio degradation characteristics of the Fischer-Tropsch gasoil fractions. Increasing the relative concentration of mono-methyl isoparaffin to other isoparaffins may improve the bio degradation characteristics of the Fischer-Tropsch gasoil fractions beyond the bio degradation characteristics of the Fischer-Tropsch-derived gasoil feedstock. 

1. A process for preparing a purified Fischer-Tropsch gasoil fraction, comprising: a) fractionating a Fischer-Tropsch-derived gasoil feedstock, wherein said Fischer-Tropsch-derived gasoil is a fluid comprising paraffins, including isoparaffins and normal paraffins, with alkyl chain lengths in the range of from 7 to 30 carbon atoms, comprising at least 70 wt % of Fischer-Tropsch-derived paraffins having 9 to 25 carbon atoms based on the total amount of Fischer-Tropsch-derived paraffins, into two or more Fischer-Tropsch gasoil fractions having a different boiling point range, wherein at least one Fischer-Tropsch gasoil fraction is a contaminant-enriched Fischer-Tropsch gasoil fraction, which is enriched in one or more contaminants with respect to the feedstock; b) providing the contaminant-enriched Fischer-Tropsch gasoil fraction to an absorption zone comprising at least one absorbent material and contacting the contaminant-enriched Fischer-Tropsch gasoil fraction with the absorbent material to absorb at least part of contaminant; and c) retrieving from the absorption zone a purified Fischer-Tropsch gasoil fraction, which purified Fischer-Tropsch gasoil fraction is contaminant-depleted with respect to the contaminant-enriched Fischer-Tropsch gasoil fraction, and wherein at least part of the contaminants in the contaminant-enriched Fischer Tropsch fraction are formed during the fractionation of step (a).
 2. The process according to claim 1, wherein at least part of the contaminants formed during the fractionation of step (a) are the product of a thermal oxidation of one or more hydrocarbon compounds in the Fischer-Tropsch-derived gasoil feedstock.
 3. The process according claim 1, wherein at least part of the contaminants formed during the fractionation of step (a) are oxygenates that are the product of a thermal oxidation of one or more hydrocarbon compounds in the in the Fischer-Tropsch-derived gasoil feedstock.
 4. The process according to claim 3, wherein the Fischer-Tropsch-derived gasoil feedstock contains one or more unsaturated aliphatic hydrocarbon compounds and at least part of the oxygenates are the product of a thermal oxidation of one or more of the unsaturated aliphatic hydrocarbon compounds.
 5. The process according to claim 1, wherein at least part of the contaminants formed during the fractionation of step (a) are unsaturated hydrocarbon compounds that are the product of a thermal dehydrogenation of one or more hydrocarbon compounds in the in the Fischer-Tropsch-derived gasoil feedstock.
 6. The process according to claim 1, wherein the one or more contaminants are selected from the group consisting of oxygenate compounds and aromatics.
 7. The process according to claim 1, wherein the absorbent material is a molecular sieve material.
 8. The process according to claim 1, the absorption zone comprises two or more absorbent materials.
 9. The process according to claim 1, wherein the contaminant-enriched Fischer-Tropsch gasoil fraction is contacted with the absorbent material at a temperature in the range of from 0 to 150° C.
 10. The process according to claim 1, wherein the contaminant-enriched Fischer-Tropsch gasoil fraction is contacted with the absorbent material in a fixed bed reactor comprising at least one fixed bed of absorbent material.
 11. The process according to claim 1, wherein the contaminant-enriched Fischer-Tropsch gasoil fraction has a final boiling point of no more than 260° C.
 12. The process according to claim 1 wherein the contaminant-enriched Fischer-Tropsch gasoil fraction has an initial boiling point of more than 260° C.
 13. The process according to claim 1, wherein at least one contaminant-enriched Fischer-Tropsch gasoil fraction has a Saybolt number below
 30. 14. The process according to claim 1, wherein the Fischer-Tropsch-derived gasoil has a final boiling point of no more than 450° C.
 15. A method of using a purified Fischer-Tropsch gasoil fraction obtained in the process of claim 1 as a solvent, diluent or functional fluid. 